Sex IS Gender. It's like saying there's a difference between your height and the cumulative units of distance from your heel to the top of your head. Nigga, they are the same.
Imagine you take 100 normal human beings, 50 men and 50 women and you tell them
1) Divide yourself along gender
2) Divide yourself along sex
3) Choose whether you want to divide the group along either gender or sex
What will happen for the first two? 50 men on one side, 50 women on the other. Now with option 3, what will happen? A bunch of confused people wondering why someone asked them whether they wanted to pick Option A or Option A. Because they are the same.
Yes, yes, 0.1% of the population suffer from a mental disease that makes them believe its not the case. Another 0.1% are fetishists/transtenders. Those are aberrations, not the norm, and you don't start saying that 1 + 1 does not equal 2 for 99.8% of the population because of a mentally ill population screaming about it on the internet.
XY makes a man. XX makes a woman. Your sex is your gender, because those two are one and the same. No amount of tumblr will ever change that.
Earlier when I said that language is defined by usage, this is what I mean.
It's incredibly hard to dictate language. Sometimes people try, but it's not usually very effective. How people actually use words in real life is what really determines their meaning. Dictionaries merely document language, they don't define it.
For example, the
Royal Spanish Academy regulates the Spanish language. Let's say they produced a document saying "this is spanish, nothing else is correct spanish". Would that mean that when Americans use the word "Spanish" to refer to Mexican Spanish, it's incorrect? No, it wouldn't. Try as they might, I doubt they'd persuade any significant amount of English speakers to change their mind, and dictionaries would continue to define Spanish as something like "the language of Spain, Mexico and many countries in Latin America".
I think you can see how this relates to you outright declaring gender to be the same as sex.
I would argue that a well passing transperson is effectively a man / woman if that's what most people see them as, IRL.
If you can take a random bystander, point to the transwoman and say "what is that person's gender?", and get them to answer "woman", then that's what they are. And not merely theoretically, but
that's actually what they are, because that's what the word means to the bystander.
Of course, there's the issue of bedroom/sex issues. They can be really important in some circumstances, but in almost all everyday social situations, I don't think they matter too often.
(I kinda really oversimplify linguistics in this post. There's all sorts of overlapping things to consider, like dialect and slang and creoles and stuff like that. But I think the basics are on target.)